Psalm 94:4

““They answer, and will speak wickedness, they all will speak that work unrighteousness” (ver. 4).” — the author cites it

“For those who are either unable to understand our arguments, or are so hardened by the habit of contradiction, that though they understand they cannot yield to them, reply to us, and, as it is written, “speak hard things,” and are incorrigibly vain.” — editor's note: Ps. xciv. 4.

Cited together in The City of God II.1.